This invention relates to a spectrum spread ("SS") reception of an SS modulated signal and, more particularly, to an SS receiver and to an SS receiving method.
In the manner which will later be described in greater detail, an SS communication network comprises an SS transmitter and an SS receiver. In the SS transmitter, adjacent symbol data of a transmitter input signal are first differential modulated into differential modulated components of a differential modulated signal. For transmission to such SS receivers as SS modulated components of an SS modulated signal through a propagation path which may be either a radio path or a wired path, the differential modulated components are multiplied by an SS mode representative of a synchronization signal which simulates noise. The SS receiver comprises a receiver unit for receiving the SS modulated signal as SS reception components of an SS reception signal, an SS demodulator unit for individually multiplying the SS code on the reception components to produce SS demodulated components of an SS demodulated signal, and a differential demodulator unit for differentially demodulating the SS demodulated components into differential demodulated components of a differential demodulated signal for use as a receiver output signal which is a reproduction of the transmitter input signal.
The reception components have a common symbol period of length. The SS demodulated components depend on correlation values between the SS code and individual components of the reception signal. The SS code consists of a predetermined number of chips having a common chip period or length which is equal to the symbol period divided by the predetermined number.
Because of SS modulation by the SS code, the SS modulated signal is hardly adversely affected by noise which spreads away during SS demodulation even if superposed on the SS modulated signal in the propagation path. The SS modulated signal furthermore has a high degree of secrecy. SS communication is consequently suitable for application to a mobile communication network and to a local area network (LAN). In fact, an excellent wireless LAN receiver station is disclosed by van Driest and another of the Netherlands, assignors to NCR Corporation, in U.S. Pat. No. 5,128,960.
In the manner which will also be later described in more detail, the differential demodulator unit is operable in a conventional SS receiver in response to a synchronizing section output signal of a synchronizing section to monitor the correlation values to determine synchronous points at time points of maximum correlation values. The synchronizing section output signal indicates successive synchronous points. Responsive to the synchronizing section output signal, the differential demodulator unit uses the SS demodulated components at the synchronous points in dealing with difference calculation.
The SS receiver is operable at a receiver clock frequency which is generated independently of a transmitter clock frequency used in the SS transmitter. The receiver clock frequency indicates the symbol period and the chip period and consequently determines the synchronous points and furthermore the SS demodulated components used in the difference calculation.
The receiver clock frequency is liable to a frequency shift relative to the transmitter clock frequency. If transmitted through the radio propagation path, the SS modulated signal is subject to undesirable influences caused by multipath and/or delay spread. This results in a time shift of the maximum correlation values relative to true synchronous points. As a consequence, the maximum correlation values may appear more than one chip period earlier or later than due synchronous points. The frequency shift gives rise to a deterioration in operation of the SS receiver. If present, the time shift additionally deteriorates precision and correctness of demodulation of the SS modulated signal.
It should be noted in connection with the above that the symbol period is more exactly what the SS modulated components have in common in terms of the transmitter clock frequency. The chip period is similarly what is commonly had by the chips used in the SS transmitter and receiver. Incidentally, the word "symbol" is used in the expression "symbol period" by referring to the symbol data and is different in meaning from the symbol of the data symbol used in van Driest et al.
It is usual as described in van Driest et al to use twice as high as the transmitter clock frequency as the receiver clock frequency. Use of this twice oversampling in the SS receiver is with a view to reducing adverse effects which may otherwise be caused by the frequency and the time shifts. That is, the twice oversampling makes it possible to deal with differential demodulation by merely using the symbol period after initial establishment of the synchronous points. This mere use of the symbol period, however, results in a difficulty in H/W operation and is incapable of coping with the frequency shift.